1. Field of the Invention
The present invention relates to lithography, and more particularly, to optical element damping systems.
2. Background of Invention
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays (e.g., liquid crystal displays), circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer or glass substrate.
During lithography, a wafer, which is disposed on a wafer stage, is exposed to an image projected onto the surface of the wafer by exposure optics located within a lithography apparatus. While exposure optics are used in the case of photolithography, a different type of exposure apparatus can be used depending on the particular application. For example, x-ray, ion, electron, or photon lithography each can require a different exposure apparatus, as is known to those skilled in the art. The particular example of photolithography is discussed here for illustrative purposes only.
The projected image produces changes in the characteristics of a layer, for example photoresist, deposited on the surface of the wafer. These changes correspond to the features projected onto the wafer during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those features projected onto the wafer during exposure. This patterned layer is then used to remove or further process exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface, or in various layers, of the wafer.
Step-and-scan technology works in conjunction with a projection optics system that has a narrow imaging slot. Rather than expose the entire wafer at one time, individual fields are scanned onto the wafer one at a time. This is accomplished by moving the wafer and reticle simultaneously such that the imaging slot is moved across the field during the scan. The wafer stage must then be asynchronously stepped between field exposures to allow multiple copies of the reticle pattern to be exposed over the wafer surface. In this manner, the quality of the image projected onto the wafer is maximized.
Conventional lithographic systems and methods form images on a semiconductor wafer. The system typically has a lithographic chamber that is designed to contain an apparatus that performs the process of image formation on the semiconductor wafer. The chamber can be designed to have different gas mixtures and/or grades of vacuum depending on the wavelength of light being used. A reticle is positioned inside the chamber. A beam of light is passed from an illumination source (located outside the system) through an optical system, an image outline on the reticle, and a second optical system before interacting with a semiconductor wafer.
A plurality of reticles is required to fabricate a device on the substrate. These reticles are becoming increasingly costly and time consuming to manufacture due to the feature sizes and the exacting tolerances required for small feature sizes. Also, a reticle can only be used for a certain period of time before being worn out. Further costs are routinely incurred if a reticle is not within a certain tolerance or when the reticle is damaged. Thus, the manufacture of wafers using reticles is becoming increasingly, and possibly prohibitively expensive.
In order to overcome these drawbacks, maskless (e.g., direct write, digital, etc.) lithography systems have been developed. The maskless system replaces a reticle with a spatial light modulator (SLM) (e.g., a digital micromirror device (DMD), a liquid crystal display (LCD), or the like). The SLM includes an array of active areas (e.g., mirrors or transmissive areas) that are either ON or OFF to form a desired pattern. A predetermined and previously stored algorithm based on a desired exposure pattern is used to turn ON and OFF the active areas.
Conventional SLM-based writing systems (e.g., Micronic's Sigma 7000 series tools) use one SLM as the pattern generator. To achieve linewidth and line placement specifications, gray scaling is used. For analog SLMs, gray scaling is achieved by controlling mirror tilt angle (e.g., Micronic SLM) or polarization angle (e.g., LCD). For digital SLMs (e.g., TI DMD), gray scaling is achieved by numerous passes or pulses, where for each pass or pulse the pixel can be switched either ON or OFF depending on the level of gray desired. Because of the total area on the substrate to be printed, the spacing between active areas, the timing of light pulses, and the movement of the substrate, several passes of the substrate are required to expose all desired areas. This results in low throughput (number of pixels packed into an individual optical field/number of repeat passes required over the substrate) and increased time to fabricate devices. Furthermore, using only one SLM requires more pulses of light or more exposure time to increase gray scale. This can lead to unacceptably low levels of throughput.
Maskless lithography systems require utilizing a minimum number of pulses to achieve dose in order to meet reasonable throughputs. Hence, it is not possible to take advantage of 50 pulse averages as in conventional lithography systems to achieve acceptable laser pulse-to-pulse variations. Conventional lithography systems use 30-50 pulses to write each feature. Typically, in maskless lithography 2-4 pulses are used to write each feature for reasonable throughput. There is a reduction in the ability to average pulses, which requires that lens cells within lithographic tools do not distort imaging.
Lens cells for lithographic tools are designed to be connected to the rest of the projection lens or projection lens body with stiff, yet thermally relieved connections. The connection must be stiff in order to minimize relative motion of the lens to the body to maintain extremely critical alignment. The same connections must be thermally relieved (kinematic) so that the element is not over constrained, which would cause distortion of the ultra high precision lens as even minute temperature changes occur. The lens shape must be preserved at nanometer level in spite of the rigid connection.
Lens mounts are made out of metal flexure blades or metal spring arrays, which allow rigid connection without optical distortion, as will be known by individuals skilled in the relevant arts. Inherently, these mounts have extremely small amounts of damping in them. The result is that a lens may experience large (at nanometer scale) amplitude motion if excited at the resonance of the mount.
In current scanning lithography system these high frequency vibrations are smeared out by the scan averaging of the system, as explained above, and do not result in measurable printing errors at the wafer. Conversely, steppers and machines operating with extremely small amounts of scan averaging, such as optical maskless lithography systems, will experience pattern placement errors due to the high frequency vibration of some of the optical elements.
The placement error amount depends on the sensitivity of each of the optical elements to vibration. Optical designs constrained to having little or no sensitivity to motion of any of the optical elements relative to each other are not practical. On the other hand, optical designs with large amounts of damping in the lens mounts are not stable due to hysteresis. Aligned elements do not spring back to the optimal aligned position after a transient disturbance, such as, shipping vibrations, if there is any substantial amount of damping in the system.
What are needed are optical systems and lens mounts that provide a balance between being over-constrained and having too much damping.